TEXES Sensitivities and Overheads

Sensitivities

Because TEXES is background photon noise limited and both the instrumental response and the background flux depend on wavelength, the instrument sensitivity depends strongly on wavelength and spectral resolution. Proposers are encouraged to contact TEXES team members for sensitivity estimates for observations at specific wavelengths, but can make initial estimates based on the table below.

The table gives sensitivities for the three spectral resolution modes after smoothing over one spectral and spatial resolution element. Sensitivity numbers in the table assume a transparent atmosphere. The noise increase due to atmospheric emission can be estimated as sqrt(1+emiss/0.1)/transmission, where emiss = 1 - transmission. PIs should generate a spectrum of the atmospheric transmission on Mauna Kea (use altitude of 13,796 feet, latitude 30 or 9, and typical "water vapor overburden"~1000-3000 microns) to estimate the emissivities at their wavelengths of interest. Sensitivity also can degrade by as much as a factor of 2 at wavelengths on grating order boundaries and edges of filters.

Surface brightness sensitivity values are given for a flux for which S/N = 5 in 1 minute total clock time. Point-source sensitivity values are given for a flux for which S/N = 5 in 1 hour total clock time. See footnotes for more details. Note that for point sources TEXES normally peaks up on a target during acquisition and this would be difficult on a source giving only S/N = 5 in 1 hour. To observe sources this faint either the source must be optically visible, so it can be placed on the TEXES "hotspot", or there must be a nearby bright infrared source from which to offset.

Resolution mode

Resolution1

Surface2brightnesserg / (s cm2 sr cm-1) 5 σ 1 min

Line3brightnesserg / (s cm2 sr)5 σ 1 min

Point source4flux density (mJy)5 σ 1 hr

Point source5magnitude5 σ 1 hr

10 µm

20 µm

8 µm

12 µm

20 µm

low

0.003µm

6 x 10-3

3 x 10-3

30

60

8.5

7.2

5.5

medium

24 km/s

1.5 x 10-2

2 x 10-3

70

140

7.6

6.3

4.6

high

3.6 km/s

6 x 10-2

8 x 10-4

250

500

6.2

4.9

3.2

1 Low-res wavelength resolution and high-res velocity resolution are approximately constant with wavelength. Medium-res velocity resolution is roughly constant, but varies with wavelength within the grating orders.2 Spectrally and spatially resolved surface brightness sensitivity is approximately constant with wavelength, assuming a transpararent atmosphere. Sensitivities are for 1 sigma 1 second per spatial resolution in a scan map, or 5 sigma in 1 minute of clock time per spatial resolution element, including time for overheads and sky measurements. For one position in a spatially extended source observed by nodding the telescope off of the source, 2 minutes of
clock time are required for a 5 sigma detection.3 Spatially resolved line brightness sensitivity for a narrow line is approximately constant with wavelength, assuming a transpararent atmosphere. Units are erg/(s cm2 sr) or 1x10-3 W/(m2 sr). Numbers are for 1 sigma in 1 second spent per spatial resolution element in a scan map, or 5 sigma in 1 minute clock time per spatial resolution element, including time for overheads and sky measurements.4 Spectrally resolved point-source flux sensitivity varies approximately linearly with wavelength, assuming a transparent atmosphere. Numbers are for 5 sigma in 1 hour of clock time for a point source nodded along the slit, and include light loss at the slit.5 Point source magnitudes for 5 sigma detections in 1 hour of clock time, assuming a transparent atmosphere.

Overheads

Overheads with TEXES associated with acquisition and telescope nodding are similar to those of Gemini facility instruments. Acquisition of bright targets requires about 15 minutes, whereas faint targets that require offsetting are likely to require about 20 minutes. Prospective users should consult with members of the TEXES team regarding their particular acquisition procedure. Re-acquisitions, which should take about half of the above times, are likely to be needed about once per hour. These overheads must be included in the time request.

Note that unlike the case for other Gemini instruments, PIs must include time for telluric standards in their TEXES proposals. For more information see here.